Electron discharge storage device



ELECTRON DISCHARGE STORAGE DEVICE Filed May 19, 1955 3 Sheets-Sheet 1FIG. I

CA THODE GLASS ENVELOPE CONDUCT] l/E (0A T/NGS INSULA TOR I STORAGE E ar 1o /z II I ELECTRON au/v COLL/MA TING LENS ELECTROSTATIC SHIELD GRIDFIG. 2A

DIELECTRIC FIRST CROSSOVER I POINT o T Z UNITY RAT/0 LINE H r T i 1 a V0Y 1 cmusaron POTENTIAL (1 (1 CATHODE POTENTIAL BY dwbiqfl ATTORNE V 1958w. KIRKPATRICK 2,859,376

ELECTRON DISCHARGE STORAGE DEVICE Filed May 19, 1955 3 Sheets-Sheet 2F/G. 4 @7000 v. @2100 v.

READ m 1/ m /5 /7 1a ourpur J3 34 I 20 E 32 I g E I i WRITE uvpur Al 3/50 V. 0 1/003 oErLEcrlo/v CONTROL PULSE 1% x AMP INPUT aE/y.INFORMATION SOURCE 1F 46 47 PULSE H GEN. 40

FIG. 5 M T I & BACK PLATE 24 4 6' 250 I" I g 200 f g g 4 I50 'fDIA-LECrR/C E SURFACE as E, 50 I6 o 8 TIME U) g 250 P METAL EACKPLAT' 24\J 200 1 I q I50 52 ,x-- '5 DlELEC7'R/C SURFACE 25 E 50 if, e x l o o As7- TIME CHARGE ON suREAcE 25 INVENTOR W. E. KIRKPATRICK A TTORNEV 1953w. E. KIRKPATRICK 2,859,376

ELECTRON DISCHARGE STORAGE ozvma Filed May 19, 1955 3 Sheets-5heet 3 FIG6 as A 59 fi/ v I T PULSE GENERATOR 46 CLIPPER 47 AMPLIFIER 4a vou's 0 kT FIG. 8

r VPEAK T (v; 4) c z,- WAX.) Q FIG 9A R 250 20o ;MTAL BACK PLATE 24 g fiV 1: I00 2 DIELECTRIC SURFACE as E u l k I a o L mus-- Z? '5 250 FIG..98 g 200 METAL BACK PLATE 24 q we I Q 5 I00 g 6.12 DlELECTR/CSURFACE'ZS 5O V0 YE o 'J L -I-L 42-r{ ---J 8 TIME INVENTOR W. E.KIRKPATRICK ATTORNEY United States Patent ELECTRON DISCHARGE STORAGEDEVICE William E. Kirkpatrick, Chatham, N. J., assignor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application May 19, 1955, Serial No. 509,534

12 Claims. (Cl. 3158.5)

This invention relates to electron discharge storage devices and moreparticularly to such devices having two stable potential states.

There are a number of storage tubes known which utilize the equilibriumpotentials acquired on insulating surfaces under electron bombordment tostore binary information. Certain of these devices and their generalcharacteristics are set forth in Storage Tubes and Their BasicPrinciples" by M. Knoll and B. Kazan, John Wiley and Sons, N. Y., 1952.Another type of storage device is that known as the dielectric islandstorage tube described in Patent 2,726,328, issued December 6, 1955, ofA. M. Clogston and in French Patent 1,055,818, October 21, 1953.

One characteristic of a dielectric island storage tube is thatinformation is stored on discrete dielectric spots mounted on a metallicback plate by placing the spots at either of two stable equilibriumconditions, either at the cathode or zero potential V or the collectorpotential V these two stable equilibrium points being on opposite sidesof the so-called cross-over potential so that, under bombardment by anelectron stream, the dielectric will charge to one or the other of thesetwo potentials.

The writing of information in the storage thus comprises pulsing thetarget while applying an electron beam and allowing the change in chargeon the surface of the dielectric to place the dielectric at one or theother of these stable potentials. However, a change in charge in volvesa finite time. This time will be dependent on the rate at which thecharge is changed and more specifically on the magnitude of the current,or electrons, either flowing towards or away from the dielectricsurface, depending on whether the secondary emission factor is less thanunity, as when writing down to V or greater than unity, as when writingup to V Further, the collector voltage V may be set at substantially anydesired value by biasing the target assembly. The cross-over voltage Vhowever, is a factor of the dielectric surface itself and cannot bevaried. If the back plate voltage V is of the order of twice or more thecross-over voltage V the charging of the dielectric surface down to thecathode or zero potential V has priorly been attained in a large numberof charging steps. Even in cases where the collector voltage is lessthan twice the cross-over voltage, as in the above-mentioned applicationand patent, two charging steps have been required.

It is, however, of great importance to be able to write or alterinformation in the tube storage as rapidly as possible. Prior techniquesof writing up to the collector potential V have been generally adequatebut prior techniques for writing down to cathode or zero potential Vhave been very time consuming, requiring as much as several times theperiod of time for the other writing operation. This has accordinglyslowed down the whole cycle of information storage to a great andundesirable degree.

It is an object of this invention to provide an improved electrondischarge storage device capable of storing in- "ice 2 formation ineither of two stable potential states on a dielectric surface.

It is another object of this invention to decrease the time for writinginformation in an electron discharge storage device and specifically ina dielectric island tube.

It is a further object of this invention to reduce to a minimum the timerequisite for erasing one value of stored information or writing down tothe other value of stored information represented by cathode potentialV,,.

It is a still further object of this invention to provide an improvedwriting circuit for an electron discharge storage device.

It is still another object of this invention to provide an improvedmethod for altering the potential state of a dielectric element in anelectron discharge storage device.

These and other objects of this invention are attained in a specificembodiment thereof wherein the writing circuit comprises a pulsegenerator, a clipper, and an amplifier arranged to provide a pulse ofspecified initial amplitude, subsequent slope and duration to the targetof a dielectric island storage tube. The current or secondary emissioncharacteristic of all materials under electron bombardment is basicallythe same and includes a portion, below the so-called cross-over voltageV at which the secondary emission factor is less than unity. In thisportion the target current i which represents electrons emitted by thedielectric, or circuit current toward the dielectric from the cathodeside, increases to a maximum absolute value [i d from the potential Vand then very quickly drops to zero at the potential V this is clearlyseen in the attached drawing and is further described below.

The rate of change of charge on the surface of the dielectric elementwill of course be dependent on the magnitude of the current at thetarget. Thus the maximum charging rate, and the minimum time requisitefor the change in charge, would occur if the current during the chargingprocess could be maintained at the maximum target current duringsubstantially the whole charging time.

In accordance with an aspect of this invention, this is attained byapplying to the target assembly and specifically to the metallic backplate thereof a triangular shaped pulse to write the surface of thedielectric element to the cathode or zero potential V This pulse has asharp initial drop of a magnitude such that the potential of thedielectric element is immediately placed in the region of substantiallymaximum target current. The pulse then rises in value at the raterequisite to keep the potential of the surface of the dielectric elementso that the current to the element is always the maximum target current.Specifically the time rate of charging the element is given by theexpression:

by appropriate correlation of circuit parameters in the writing circuit.Further the duration of the pulse is made such that the pulse is presentonly until the potential of the metallic back plate has been returned toits initial or stable value V at that time the pulse is effectivelyturned ijTtminli c oil and the dielectric element charges the remainderof the way down to V under the influence of the electron beam alone.

In one specific illustrative embodiment of a writing circuit, thedesired writing pulse is attained by first rapidly discharging acondenser through a gas tube and then charging the condenser through aresistor, the values of resistor and condenser being such that thecharging time constant affords the desired slope of the writing pulse.The pulse is then clipped and amplified, the amplifier being arranged sothat the pulse has the required initial amplitude to reduce the voltageon the dielectric element to the region of maximum target current.

It is a feature of this invention that a writing pulse be applied to anelectron discharge storage tube to place a dielectric element at thepotential V,,, the writing pulse being shaped to maintain the surface ofthe dielectric element at a potential for maximum current whereby aminimum time is required to change the charge on the dielectric elementand thus write the desired information thereon.

It is another feature of this invention that the writing pulse forwriting down to cathode potential V on a dielectric element in anelectron discharge storage device have an initial steep portion and arising portion, the slope of which corresponds to the rate of change ofcharge on the dielectric element. Specifically, it is a feature of thisinvention that the slope of the rising portion of the writing pulse begiven by the expression l Tlmnx)l c where H is the maximum absolutevalue of the target or charging current and C is the capacitance betweenthe surface of the dielectric element and the metallic back plate of thetarget.

It is still another feature of this invention that the initial amplitudeof the writing pulse be such as substantially immediately to change thepotential of the dielectric element from collector potential V to thepotential at which the target current is a maximum, the slope of therising portion of the pulse be as given above, and the duration of thepulse be just sufficient to raise the back plate potential back to V,,.

It is a further feature of this invention that the writing circuit for adielectric island storage tube comprise a pulse generator, a clipper,and an amplifier, the pulse generator generating a pulse having aninitially sharp front and a rising portion having a slope as givenabove, the clipper and amplifier determining the amplitude of theinitially sharp front portion to be such as immediately to drop thepotential of the dielectric element to the desired value.

It is still a further feature of this invention that the writing circuitfor a dielectric island storage tube comprise a pulse generator having acondenser, a gaseous discharge device, and a resistor, the condenserdischarging through the gaseous discharge device to give an initialsteep front to the generated pulse and charging back through theresistor, the time constant of the charging circuit being such that theslope of the generated pulse maintains the surface of the dielectricelement at the target of the tube at the potential for maximum targetcurrent during the charging and writing process.

A complete understanding of this invention and of these and variousother features thereof may be gained from consideration of the followingdetailed description and the accompanying drawing, in which:

Fig. l is a representation of electron discharge storage tube of thedielectric island type with which this invention may be employed;

Figs. 2A and 2B are partial plan and section views, respectively, of thetarget assembly of the tube of Fig. 1 showing the dielectric islands andmetallic back plate of the target assembly of the tube;

Fig. 3 is the characteristic curve for target current i against targetpotential V on bombardment of the target by an electron beam;

Fig. 4 is a schematic representation of the tube of Fig. l and a blockdiagram representation of the writing circuits therefor in accordancewith one specific illustrative embodiment of this invention;

Fig. 5 is a plot of potential against time for storing information bywriting up to the collector potential V Fig. 6 is a schematicrepresentation of one specific illustrative erasing circuit inaccordance with the embodiment of this invention depicted in Fig. 4;

Fig. 7 is a plot of potential against time for storing information bywriting down to the cathode potential V in accordance with thisinvention;

Fig. 8 is a voltage-time plot indicating the characteristics of thewriting down or erasing pulse applied to the target of the storage tubein accordance with this invention; and

Figs. 9A and 9B are plots of potential against time for storinginformation by writing down to cathode potential in accordance withprior art processes.

Turning now to the drawing, Fig. l is a representation of a storage tubewith which this invention may be employed. The tube is of the type knownas a dielectric island storage tube, as described in Patent 2,726,328,issued December 6, 1955, of A. M. Clogston and French Patent 1,055,818,October 21, 1953, and comprises an elongated glass envelope 9 in whichare positioned, in succession, an electron gun comprising a cathode 10,a control grid 11, a first accelerating electrode 12, a focusingelectrode 13, and a second accelerating electrode 14, two pairs ofdeflection plates 15, a collimating lens comprising a first conductivecoating 17 on the inner wall of the envelope 9 and electricallyconnecting to the accelerating electrodes 12 and 14 and a secondconductive coating 18, and a target assembly comprising an electrostaticshield grid 20 and a storage target 21. The shield grid 20 is a finemesh or wound grid of high electron transparency spaced, as of the orderof 0.010 inch, from the storage target 21; the grid is advantageouslyelectrically connected to the second conductive coating 18 and serves toprevent pulses from the target region affecting the action of thecollimating lens.

The storage target 21 in a dielectric island storage tube of the typedescribed in the above-mentioned United States and French patentscomprises, as seen in Figs. 2A and 2B, a metallic back plate 24, such asof copper Inconel, an alloy of nickel, chromium and iron, or othermaterials, on which are positioned a plurality of small dielectric orinsulating elements or islands 25. In one specific illustrativeembodiment the target back plate 24 was provided with an array of 250dielectric spots per linear inch. Thus the islands 25 may be of theorder of .002 inch in diameter and may be positioned on centers spaced.004 inch apart. The target 21 may advantageously be prepared byphoto-etching the metallic back plate 24 using a dot negative of therequired mesh size and subsequently filling the etched holes with asilica sol suspension of Vycor glass powder, Vycor itself being nearlypure silica. However, other insulating elements may be used for theislands 25, as is known in the art.

In Fig. 3 is shown the curve characteristic of target current i as afunction of bombarding potential V when the target is bombarded by anelectron beam. This curve is basically characteristic of all materials,both insulators and conductors, with a secondary emission ratio 6greater than unity for some range of bombarding potential V As can beseen, the characteristic crosses the current axis at three distinctpoints, namely, when the bombarding potential is at the cathodepotential V at the collector potential V and at some intermediatepotential V In the specific embodiment of this invention descibedherein, the portions of the metallic plate 24 between adjacentdielectric islands 25 serve as the collector electrode and thus thetarget potentialis also the collector potential V Accordingly, asfurther discussed below, whenever the target potential is changed thecollector potential V is also changed. It should be noted that in thisembodiment the grid is sufiiciently removed from the surface of thetarget 21 to have substantially no effect as a collector.

As can be seen, the secondary emission ratio is less than unity forbombarding potentials less than V and is greater than unity forbombarding potentials greater than V V is commonly called the firstcross-over and is the bombarding potential value at which 6:1. The valueof V as is known, is entirely a property of the bombarded material andis very susceptible to surface conditions; it cannot, however, bechange-d by varying potentials in the storage tube circuit. The targetcurrent curve, however, again crosses the axis at the collectorpotential V The exact shape of the curve at this potential depends on anumber of factors, among them being the initial velocity of thesecondary emission of the material being bombarded and the geometricalconfiguration of the target-collector arrangement. If the collectorpotential is changed, as to V the overall curve shifts to the dottedone, as shown.

As is known in the art, the points V =V and V =V are stable equilibriumpoints to -one or the other of which an insulator or unconnected metaltarget will go and remain at under the influence of a continuouslybombarding electron beam. The point V however, is a point of unstableequilibrium and small departures from the potential Vu in eitherdirection will cause the target to seek that one of the two stablepoints towards which the departure is. In some prior types of storagetubes, such as that known as the barrier grid storage tube, storage isattained only in the neighborhood of the collector potential V which maybe of the order of twenty times the voltage of the cross-over orequilibrium point V In such tubes the target is a continuous plate of adielectric material positioned on a metallic backplate and the potentialof the dielectric or storage surface is always more positive than V0.Information is stored by applying a signal to the back plate while thebeam is impinging on the target so as to leave a charge on thedielectric surface when the beam and signal are removed. In the processof reading out the information, the stored signal is destroyed.

However, in a dielectric island storage tube both stable equilibriumpoints V and V are employed for binary storage of'information', V beingof the order of only a few times the magnitude of V In dielectric islandtubes the process of reading does not destroy the information stored butin fact the impinging beam revivifies the potential of the dielectricspot from any change'which may have occurred for any reason whatever.Thus in the dielectric island type of storage tube one value of binaryinformation, which may be defined as binary 1," is written or storedwhen the dielectric island is at the potential V and the other value ofbinary information, which may be defined as binary O, is written orstored when the dielectric island is at the potential V These aresometimes referred to as writing up to V and erasing or writing down" toV While the above and following descriptions are concerned with thecharacteristics of a single dielectric island 25, considered exemplaryof the action of the target21, it is to be understood that in actualpractice the electron beam will normally impinge on a number of suchislands 25 simultaneously, the dielectric island diameter and theseparation of the islands being small in comparison to the electron beamdiameter. In practice, it has been found that if the beam diametercovers about ten to twelve dielectric islands, the target appearssubstantially as a homogeneous surface to the electron beam. In thismanner the acute problem of registration 6 that would occur were itnecessary to focus an electron beam cyclically on but a singledielectric island is avoided.

In Fig. 4 there is depicted, in schematic form, a dielectric islandstorage tube as described above with reference to Figs. 1 and 2,together with the writing circuits therefore in accordance with oneillustrative embodiment of this invention. As there depicted the target21 is connected through a resistor 30 to a source 31 of positivepotential, which in one specific illustrative embodiment was +150 volts.The target 21 is also connected through a condenser 32 to the output andwriting circuits. The output circuit comprises merely an output terminal34 connected to the capacitor 32 through an output amplifier 33, theread output pulse being transmitted from the target 21 through thecapacitor 32 to the amplifier 33 and thence, amplified, to the outputterminal 34. The reading operation is described further below.

The writing circuit comprises an input information source 36 which, inaccordance with the dictates of associated circuitry, initiates either awrite 1 pulse 37 or a write 0 or erase pulse 38. Let us consider theoperation of writing a 1" or writing up to the potential V The pulse 37is applied to a pulse generator circuit 4t) which generates a squarepulse 41. Pulse 41 is applied through the capacitor 32 to the backplate24 of the target 21.

To appreciate best the operation of a dielectric island storage tube inwriting up to V or in changing a stored 0 to a stored 1 let us againconsider the characteristic of Fig. 3 together with the voltage-timeplot of Fig. 5 and let us assume, for purposes of this explanation, afew exemplary values. Thus we shall assume a crossover voltage V ofvolts, a collector voltage V of 150 volts and a writing pulse 41 ofvolts amplitude; the duration of the pulse is not critical provided thatit is of a minimum length, as will be apparent.

Initially with a zero stored in the target 21 the potential of thedielectric island 25 is V or zero and the potential of the metalbackplate 24 is V or volts. These conditions are indicated at the leftof Fig. 5 wherein the continuous line 43 indicates the potential of thedielectric island 43 and the dashed line 44 the potential of the metalbackplate 24. As these two potentials are stable equilibrium conditions,they will not change even under electron bombardment of the target 21.

When it is desired to write 1 the pulse 41 is applied to the backplate24 and the potential of the backplate is raised from V =150 volts to V':250 volts. As the surface of the dielectric island 25 is capacitivelycoupled to the back plate 24 it also must rise in potential lOO voltsfrom the potential V O to a potential of 100 volts. However, as isreadily seen on both Figs. 3 and 5, the potential of the dielectricisland is now above or to the right of V and therefore in an unstableregion where it will tend to go to V under bombardment by an electronbeam. Accordingly under bombardment by an electron beam the potential ofthe dielectric island will rise to V :25O volts. Then when the pulse 41is terminated, any time after suflicient charge has accumulated on thedielectric island 25 to raise its potential to 250 volts, the potentialof both the backplate 24 and the surface of the dielectric island 25fall back to V :l50 volts, which is the stable equilibrium voltage forstorage of a 1 in this specific embodiment.

In order to increase the capacity and usefulness of the storage tube itis desirable to reduce the time requisite for writing and erasing storedinformation. It is therefore desirable to have the surface of thedielectric island, on writing a "1, charge under the bombardment of theelectron beam at a fast rate, i. e., with a large value of targetcurrent i This is readily attained by employing a square shaped pulse 41which rapidly causes the potential on the dielectric island surface toincrease to values which, as seen in Fig. 3, correspond to large valuesof target current i When it is desired to write or to erase a 1, thewrite 0 pulse 38 is applied from the input information source 36 to theerase circuit which comprises a. pulse generator 46, a clipper circuit47, and an amplifier 48. The pulse generator 46 is triggered by theinput pulse 38 and produces a pulse 50 having an initial steep frontfollowed by a slower rising portion. This pulse is applied to theclipper circuit 47 which removes all but the lower section of the waveshape 50, producing the pulse 51. Pulse 51 is then applied to theamplifier 48 the constants of which have been predetermined so that theoutput pulse 52 therefrom is of the same shape as pulse 51 but of anamplitude suitable for use in erasing the information on the target 21,as discussed further below.

The pulse generator 46 may advantageously include a condenser 54, asseen in Fig. 6 which is a detailed schematic diagram of one specificillustrative embodiment of the erase circuit in accordance with Fig. 4.The wave form 50 may thus be attained by a rapid discharge of thecondenser 54, as through a gaseous discharge device 55, followed by acharging of the condenser 54 through a resistor 56, the values ofcapacitance and resistance of the condenser 54 and resistor 56 beingchosen so that the pulse 50 has a predetermined wave form in accordancewith an aspect of this invention, as discussed further below.

ln the specific embodiment of the erase circuit depicted in Fig. 6 apositive going pulse 50 is applied to the clipper circuit 47, whichadvantageously comprises a pentode 58 which both clips and inverts thepulse, only the portion E of pulse 50 being employed. The pulse 51 fromthe pentode plate is applied to the grid of a triode 59 of the amplifiercircuit 48, the desired erase pulse 52 being taken from the cathoderesistance 60 of the triode.

In order best to understand the erase operation in accordance withaspects of this invention, let us again consider the characteristiccurve of Fig. 3 together with the voltagetime plot of Fig. 7, assumingthe same exemplary values as in the writing operation described abovewith reference to Fig. 5. When it is desired to erase a stored bit, i.e., to write down from a stored l, at V,,, to a stored 0, at V both themetal backplate and the surface of the dielectric island are initiallyat the potential V which we have assumed to be 150 volts; in Fig. 7 thecontinuous line 63 indicates the potential of the dielectric islandsurface and the dashed line 64 the potential of the metal backplate 24.

When the erase pulse 52 is initially applied from the erase circuit, andspecifically from the amplifier 48 thereof, to the backplate 24, thepotential of both the backplate 24 and the dielectric island 25 drop tosome value below the cross-over potential V Accordingly, when anelectron beam is applied to the dielectric island, the surface of thedielectric will tend to charge towards the stable equilibrium point V:0. in this process of erasing the surface from V to V a finite time isrequired to make this change in potential due to change of charge on thedielectric island. In accordance with an aspect of this invention, thischarging time is reduced to a minimum.

Let us turn back to Fig. 3; in this curve it will be observed that thereis a region A close to the cathode potential V at which the currentleaving the target. or the electrons arriving at it, is a maximum. Thisis the region then at which charge accumulates most rapidly in loweringthe potential of an element towards cathode potential. In accordancewith an aspect of this invention, a dielectric island at potential V maybe most rapidly reduced to V by first pulsing the spot to the A regionand then allowing the pulse to decay at the rate the spot chargesnegatively while it is in the A" region. Accordingly the parameters ofthe pulse 52 are chosen, by the capacitor 54 and resistor 56 of thepulse generator circuit 46 and the amplifier 48, so that the initialamplitude of the pulse will drop the voltage of both the backplate 24and the dielectric island 25 to the voltage A at this A" region and thenkeep the dielectric island surface at that potential while the backplatepoten tial rises again to the potential V This action is depicted inFig. 7. The dielectric island 25 and the target backplate 24 areinitially at the potential V =l5tl volts, which is of course above thecrossover V When the pulse 52 is applied, the voltage of both thebackplate 24 and the dielectric island surface, which are capacitivelycoupled together, drop to the potential A of region A." In this regionthe time rate of charging, in the negative direction, is:

where iflmax) is the absolute value of the target current i in the Aregion and C is the capacitance of the spot whose potential it isdesired to change.

In accordance with an aspect of this invention the backplate pulse 52 isshaped to rise at the same slope, that is, the pulse slope is made equalto 0 Accordingly the potential at the surface of the dielectric island25, as indicated by the line 63, remains at the potential A. When thepotential of the backplate 24 has been raised, by the pulse 52, to V,,,the pulse is removed and the dielectric island surface will then chargethe rest of the way to the potential V =0, as indicated in Fig. 7.

The pulse generator 46 in the erasing circuit generates a pulse, as bycharging of a condenser, whose rising slope is given by the expression MC for the particular target 21 of the storage tube. In this manner thetarget dielectric island surface remains at the potential at whichmaximum current leaves the surface during substantially the entireerasing operation, thereby attaining the fastest erasing process. Ingeneral the potential will be of the order of 5 or 10 volts, dependingon the material of the dielectric.

The action of the target 21 during the erasing process can also beconsidered by looking more carefully at the potential of the surface ofthe dielectric island 25, which is given by the line 63 in Fig. 7. Atany instant the potential at the surface of the dielectric 25, in termsof the cathode potential, will be determined by the potential on thebackplate 24, to which the surface 25 is capacitively coupled, and thecharge that has built up on the surface. Accordingly we can consider theline 67 as being the potential of the charge on the surface 25 due tothe charging action of the beam. The rate of charging, and thus theslope of this line, are given by Equation 4. Further in accordance withthis invention line 67 is of maximum slope as the charging action occursthroughout substantially its entire duration with maximum current. Atany instant the sum of the charge voltage, as given by line 67, and thebackplate voltage, as given by line 64, is the potential of thedielectric surface, as given by line 63, and specifically is thepotential A.

The characteristics of the pulse 52 in accordance with this inventionare indicated in Fig. 8 and are as follows:

(1) The initial amplitude of the pulse is V A;

(2) The slope of the pulse is l rtmml C (3) The duration of the pulse is(V -MC A further appreciation of the advantages of this inand 9Btogether with the current characteristic of Fig. 3. Fig. 9A is avoltage-time plot showing the effect of attempting to erase with asingle square erase pulse when the target potential V is more than twicethe cross-over potential V In this case the backplate 24 and dielectricisland surface are initially at V,,. When the target is pulsed both thebackplate potential 631 and the dielectric island surface potentials 641drop, because of the pulse, to some value below V At this potential,under the influence of an impinging electron beam, the potential of thedielectric island surface will charge down to V =0. When the pulse isremoved, the potentials of both the backplate and the dielectric islandwill increase by the magnitude of the pulse. As V V :l00 volts and V =50volts, in this exemplary embodiment, the dielectric potential mustreturn above V by some value, after which it will charge back to V andthus no erasing is possible by this process. This process however can beutilized if V is chosen so that V V V as is disclosed in theabove-mentioned patents.

For the exemplary embodiment depicted a stepped erase pulse could beemployed, as depicted in Fig. 9B wherein line 632 indicates thepotential of the backplate 24 and dashed line 642 the potential of thesurface of the dielectric island. By arranging each increment or step ofthe rise in backplate potential less than V the potential of thedielectric island surface 25 is never raised above V and thus alwayscharges back to the potential V In this manner erasure of information isattained in successive steps. It is apparent however that such is a verytimeconsuming process.

in accordance with this invention erasure of informa tion or thechanging of the dielectric island surface potential from V,,corresponding to a stored binary l, to V corresponding to a storedbinary "0, is attained in a single step in the minimum time by properchoice of the shape of the erase pulse applied to the backplate in termsof the physical constants of the target itself.

When it is desired to read the stored information the electron beam isreturned to the dielectric islands. If a zero is stored in thedielectric island, the dielectric surface will be at the potential V,,:0and will tend to restrain the emission of secondary electrons from themetal backplate portions immediately surrounding the dielectric island;if a binary 1 has been stored, the dielectric surface is at thepotential V which is the same as that of the backplate, and this actionwill not occur. Thus while from all dielectric spots, either at V :0 orat V a current equal to the impinging beam current is returned (see Fig.3), the secondary current from the metal area around the dielectricspots that escapes and is not returned to the target varies from a highvalue from the regions around spots at V where the secondaries areuninhibited in their flow, to a low value from the metal areas aroundthe spots at V where secondaries tend to be suppressed. This variationin effective secondary emission of the metal target plate is a usefulform of signal separation which reveals itself as an amplitude variationin the output pulse appearing at output terminal 34 during the readingprocess.

It is to be understood that the above-described arrangements are merelyillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge storage device comprising a target having adielectric element and a metallic backplate, said dielectric having acurrent voltage characteristic under bombardment by an electron streamhaving a maximum absolute value of target current adjacent zeropotential, means for projecting a stream of electrons against thesurface of said dielectric element, and means for maintaining the targetcurrent at said maximum absolute value during charging ofsuid surface bysaid electron 'strearn,'sai'd last mentioned means including means forapplying a pulse't'o said backplate having an initially steep frontportion and a slowly rising portion, the slope of said rising portioncorresponding to the rate of change of charge on said surface of saiddielectric element during charging of said surface by said electronstream.

2. An electron discharge storage device comprising a target having adielectric element and a metallic backplate, there being a capacitance Cbetween the surface of said dielectric element and said backplate, meansfor projecting a stream 'of electrons against said target, saiddielectric having a current voltage characteristic under bombardment byan electron stream having a maximum absolute value of target current lim adjacent zero potential, and means for maintaining the potential onsaid dielectric surface during charging of said surface by said electronstream at a value that the target current at said surface is h d, saidlast mention means including means for applying a pulse to saidbackplate having an initially steep falling portion to drop thepotential of the surface of said dielectric element to said value andhaving a slowly rising portion having a slope li fl/C.

3. An electron discharge storage device wherein information is stored onthe surface of a dielectric element by placing said surface at one oftwo stable potentials comprising a target having a dielectric elementand a metallic backplate, there being a capacitance C between thesurface of said dielectric element and said backplatc, means forprojecting a stream of electrons at said dielectric element, means forbiasing said backplate at one of said stable values of potential V saiddielectric having a current-voltage characteristic under bombardment bysaid electron stream having a maximum value of target current i at avoltage A adjacent the other stable value of potential on saiddielectric surface during charging of said surface by said electron beamat said voltage A to attain minimum time for changing the potential ofsaid surface from said stable value V to said stable value V said lastmentioned means including means for applying a pulse to said backplatehaving an initially steep falling portion of an amplitude (V A), aslowly rising portion having a slope and a duration c T(max)- 4. Anelectron discharge storage device in accordance with claim 3 whereinsaid last mentioned means further comprises pulse generator means forgenerating a pulse having said slope and amplified means for determiningsaid amplitude of said initially steep falling portion.

5. An electron discharge storage device in accordance with claim 4wherein said pulse generator means includes a condenser, means includinga gaseous discharge device for rapidly discharging said condenser, andmeans including a resistor for slowly charging said condenser at a rateequal to said slope.

6. An electron discharge storage device wherein information is stored onthe surface of a dielectric element by placing said surface at one oftwo stable potentials comprising a target having a dielectric elementand a metallic backplate, means for projecting an electron streamagainst the surface of said dielectric element, and circuit means forapplying a pulse to said backplate to maintain the current at saidtarget constant under bombardment by said electron stream, said circuitmeans T( ax) including a pulse generator and amplifier means forgenerating a pulse having an initially falling portion and a slowlyrising portion having a slope equal to the rate of change of charge onsaid dielectric element under said bombardment by said electron stream.

7. The method of altering the potential state of a dielectric element inan electron discharge storage device comprising projecting an electronstream against said element, applying an initial voltage to a metallicbackplate adjacent to said element to establish the potential of thesurface of said element at a value for maximum electron collection bysaid element, and maintaining the potential of said surface constantduring charging of said element by said electron beam, whereby the rateof change of charge on said surface is maximum duringv the period ofsaid charging.

8. The method of altering the potential state of a dielectric element inan electron discharge storage device comprising projecting an electronstream against said element, applying an initial voltage to said elementso that the magnitude of the current at said element is maximum, andcausing said voltage applied to said ele ment to change at a rate equalto the rate of change of charge on the surface of said element, wherebythe potential of the surface of said element remains constant.

9. The method of altering the potential state of a dielectric element inan electron discharge storage device, said dielectric element beingsupported on a target backplate having an original value of potentialapplied thereto, comprising the steps of projecting an electron streamagainst said element, applying a voltage pulse to said backplate of amagnitude to determine the potential of the surface of said element atan initial value for maximum target current under bombardment by saidelectron stream, causing the magnitude of said voltage pulse to vary ata rate equal to the rate of change of charge on said dielectric elementsurface whereby the potential at said surface remains substantiallyconstant, and terminating said voltage pulse when the potential of saidbackplate has returned to its original value.

10. The method of changing the potential state of a dielectric elementin an electron discharge storage device initially at a collectorpotential V to cathode potential V comprising the steps of bombardingthe element with an electron stream, applying a pulse to said element tochange the potential of the surface of said element from V to apotential adjacent V at which the electron collection of the element isa maximum, and maintaining the potential at said surface constant duringcharging of said surface by said electron stream, whereby the rate ofchange of charge on said surface is maximum over the period of saidcharging.

11. The method of changing the potential of a dielectric element in anelectron discharge storage device initially at a collector potential Vto cathode potential V said dielectric element being mounted on ametallic backplate, comprising the steps of bombarding the element withan electron stream, applying a pulse to said backplate initially tochange the potential of the surface of said element from V to apotential adjacent V at which the electron collection of the element isa maximum, causing the magnitude of the pulse applied to said backplateto decrease at a rate equal to the rate of change of charge on thesurface of said element due to said electron stream, whereby thepotential of said surface remains constant, and terminating said pulsewhen the potential of said backplate has been raised back to V 12. Themethod of changing the potential of a dielectric element in an electrondischarge storage device initial- 1y at a collector potential V tocathode potential V said dielectric element being mounted on a metallicbackplate, comprising the steps of bombarding the element with anelectron stream, applying a pulse to said backplate initially to changethe potential of the surface of said element and the potential of saidbackplate from V to a potential adjacent V; at which the electroncollection of the element is a maximum, causing the amplitude of saidpulse applied to said backplate to decrease at a rate equal to the rateof change of charge on the surface of said element due to said electronstream whereby the potential of said surface remains constant and therate of change of charge on said surface is maximum during the period ofsaid charging, discontinuing said pulse when the potential of saidbackplate has returned to V and causing the potential of said surface tochange from said constant potential adjacent V to V under bombardment bysaid electron stream alone.

References Cited in the file of this patent UNITED STATES PATENTS2,548,405 Snyder Apr. 10, 1951 2,612,634 Mesner Sept. 30, 1952 2,706,264Anderson Apr. 12, 1955 2,726,328 Glogston Dec. 6,, 1955

